The relation of rhodopsin and perceptual threshold will continue to be investigated in normal and abnormal human retinas. Psychophysical and reflection retinal densitometric techniques will be employed. Physiological processes intervening between light absorption by rhodopsin in the rod outer segments and the perceptual threshold will be evaluated using the a- and b-wave of the electroretinographic response. The characteristics of normal a- and b-wave stimulus/response functions obtained from dark adapted eyes and after full and partial rhodopsin bleaches will be analyzed and compared to rhodopsin densities and psychophysical sensitivities obtained in parallel experiments. As the range of normal a- and b-wave stimulus/response characteristics becomes defined, selected patients will be studied in the dark adapted condition and after partial bleaches. The goal is to localize the site of pathophysiological processes which may in the future be alleviated by specific therapeutic strategies.